In many applications in optical communications it is desirable to have a single circuit that can operate over a wide variety of signal formats. This variety can require different circuitry to set the proper threshold (the level at which a decision is made whether the signal is a one or zero) for data recovery. In most transmission codes which have a zero DC component, this level is equal to the average of the signal and a simple averaging circuit is adequate. For many codes, however, like the PPM encoding used in IR LAN communications and the FDDI code in fiber optic communications, the proper decision point is not equal to the average of the signal. In these cases a common technique is to use the positive and/or minimum peaks of the signal to determine the proper threshold level.
FIG. 1 shows typical peak detect and averaging circuits. In the peak detect circuit, FIG. 1(a), with the arrival of each pulse in the signal a correction is applied through the diode, D1, to supply current to charge the capacitor, C1, until its voltage equals the peak of the signal. The differential amplifier, A1, amplifies the difference between the peak of the signal and the value on the capacitor to improve accuracy. The typical averaging circuit, FIG. 1(b), is simply a resistor and capacitor. The output, taken from the junction of the resistor and the capacitor, represents the average of the signal applied to the other end of the resistor.
It would be desirable if there were a circuit which could operate with both types of code, i.e., one that is capable of detecting both the peak and average values of a signal. Such a circuit would require less space to implement, would be cheaper to build and would require less power than separate circuits.